[TmEE]

S-video and composhit come from DC as is, there's no encoding happening... or at least there is no need. One can have external encoder in RGB mode but it will produce inferior image due to no phase information that the video DAC in DC has access to.
My VGA box has the VGA out, TRS audio out and s-video + RCA composhit and audio outs. Besides a switch there's a LED and a logic chip to buffer the sync signals.